Original Video Animation (or OVD - original DVD animation).
OVA's are episodes produced exclusively for video release, though they usually air on television a few months later. Most anime shows are released for television first (in Japan) and then released on DVD later, but OVA's are the reverse.
OVA's often contain specialextra episodes, marketed to fans who may have seen an entire series but are hungry for more.
Hey Have you seen the OVA of Gravitation(anime)it looks great.